18ElektronenRegelSystem

18ElektronenRe, also known as Eighteen Electron Rule, is a principle in inorganic and organometallic chemistry used to predict and rationalize the stability of transition metal complexes. The rule states that a stable transition metal complex typically has 18 valence electrons, analogous to the noble gas electron configuration. This electron count includes the metal’s valence electrons plus electrons donated by ligands.

The origin of the rule lies in the observation that many stable transition metal complexes tend to fill their valence shell with 18 electrons, achieving a noble gas configuration analogous to that of the preceding noble gas in the periodic table. It is most applicable to coordination complexes of transition metals in their zero or near-zero oxidation states, with ligands such as carbon monoxide, phosphines, and halides.

Calculation of the total electron count involves summing the metal’s valence electrons and the electrons donated by ligands. For example, a metal with a d^6 configuration and two monodentate ligands each donating two electrons would be counted as having 6 (metal) + 2×2 (ligands) = 10 electrons, but when considering the metal’s s and d electrons along with electrons from multiple ligands, the total can reach 18 electrons for stable complexes.

Although useful, the 18-electron rule is a guideline rather than a strict rule, and some stable complexes deviate from it. It is most reliable for describing low-spin, stable species, especially in the context of transition metal carbonyl and phosphine complexes. Its application aids in understanding the bonding, structure, and stability of various organometallic compounds.